1. Field of the Invention
This invention generally relates to the encoding and decoding of composite color video signals having separate luminance and color components and, in particular, to an adaptive technique which bases color component encoding/decoding on characteristics of the luminance signal, in order to reduce the amount of information needed to specify the composite video signal.
2. Description of the Prior Art
Even though composite color video signals may be represented in terms of separate color and luminance components, it is well known that various useful inter-relationships exist between these components that can be exploited in order to increase encoding efficiency. For example, it was shown by A. N. Netravali and C. B. Rubinstein in U.S. Pat. No. 4,023,199 issued May 10, 1977 that busy picture areas, as indicated by large luminance signal activity, would tolerate encoding errors in the chrominance component much better than would flat picture regions. Accordingly, the quantizer characteristics of the chrominance encoders were adapted to the luminance signal activity, specifically, by using a coarse quantizer in active areas, and a fine quantizer only in flat regions. This adaption technique was successful in conserving bandwidth; however, both luminance and chrominance information was transmitted for each picture element.
In a second example, described in the copending application, Ser. No. 826,025, filed Aug. 19,1977, by the present applicants, it was found that the luminance and color components, although being independent in value, exhibited certain similarities as to when changes occur, since picture edges generally contain both luminance and color discontinuities. This characteristic was used to advantage in DPCM encoding by forming a series of predictions of the present luminance sample, using various combinations of the previous luminance samples; each prediction was evaluated against the true luminance value, to determine the combinatorial pattern that yielded the best result, and this pattern was then used to combine previous color component samples to yield a prediction of the present sample. Here again, this technique worked well in reducing the prediction error in a DPCM encoder, but again had application only in systems wherein some clor information is transmitted for each picture element.
One system that altogether avoids transmission of chrominance information when such information is deemed redundant or unnecessary is described in U.S. Pat. No. 3,803,348 issued to J. O. Limb and C. B. Rubinstein on Apr. 9, 1974. In this system, advantage was again taken of the fact that significant changes in color information generally accompany significant changes in the luminance signal or, stated differently, that intensity edges usually have accompanying color transitions. Specifically, Limb et al. arranged to inhibit transmission of chrominance values until a significant luminance change occurred; only then was a chrominance value transmitted to the receiver, it being assumed in the interim that the chrominance value was an average of previous chrominance samples.
In an improvement to the Limb et al. technique, it was recognized in the C. C. Cutler, J. O. Limb and C. B. Rubinstein U.S. Pat. No. 3,860,953, issued Jan. 14, 1975, that important chrominance changes sometimes occur alone (i.e., without accompanying luminance changes). To account for this, the luminance signal was intentionally distorted to signal to the remote receiver that an updated chrominance value was arriving.
While the Limb et al. and Cutler et al. techniques were somewhat successful, it is important to note that a constant or average value was assumed for picture elements lying between elements for which significant luminance changes exist. As a result, no use was made of the luminance data between "significant" changes, and this reduced encoding efficiency. Moreover, if the threshold that determines when a luminance change is significant were to be lowered, then unnecessary chrominance information would be transmitted, again leading to a decrease in efficiency.
In view of the foregoing, it is the broad object of the present invention to increase the efficiency of encoding of color video signals, and particularly the color components thereof. Other objects are to eliminate the need to encode redundant color component information, and to thus transmit only significant changes. However, in the interim between such changes, the color component value must be updated by a selective extrapolation (or prediction) process which fully utilizes the available luminance information. In all events, the production of a subjectively pleasing rendition of the original object is, of course, essential.